1. Field of the Invention (Technical Field)
The invention has application to instruments that use optical spectroscopy with variable-wavelength light sources to monitor a known species.
2. Background Art
Optical spectroscopy is a well-established technique that allows quantitation of a known species within a sample by measuring the fraction of light intensity that is absorbed by the sample at a specific wavelength. The underlying scientific principle, known as Beer""s Law, is expressed as:
I/I0=exe2x88x92n"sgr"l,xe2x80x83xe2x80x83(1)
where I is the light intensity after passing through the sample, I0 is the initial light intensity, n is the species number density or concentration, "sgr" is the species optical absorption cross-section which is a fundamental property of the species and depends on wavelength, and l is the optical path length through the sample. Typically, "sgr" and l are well known, implying that measurement of absorbance, where absorbance is defined as xcex1=xe2x88x92loge (I/I0), is sufficient to determine n, the species number density within the sample.
Continuous monitoring of a target species concentration can be made practical through the use of continuous measurement of optical absorbance. It is instructive to focus on applications in which the light source is a wavelength tunable continuous-wave laser and the sample probed contains a gas exhibiting an absorption spectrum composed of well resolved, narrow lines. At least one of the absorption lines of the target gas is assumed to lie within the accessible wavelength tuning range of the laser. The quantity of the gas in a sample is determined by measuring the absorbance of the laser light when the laser wavelength is made coincident with one pre-selected absorption feature.
Monitoring species at low concentrations requires measuring accurately weak absorbances, i.e., xcex1 less than 10xe2x88x923. Signals due to weak absorbances are often obscured by laser noise. The dominant noise source is known as xe2x80x9c1/fxe2x80x9d noise because it decreases with increasing frequency; therefore, most strategies for improving the signal-to-noise ratio of absorbance measurements attempt to shift the detection bandwidth to high frequencies. One such approach, wavelength modulation spectroscopy, is effective for avoiding laser 1/f noise. The technique is described by Wilson (G. V. H. Wilson, xe2x80x9cModulation Broadening of NMR and ESR Line Shapes,xe2x80x9d J. Appl. Phys. 34, 3276-3285 (1963)) and by Arndt (R. Arndt, xe2x80x9cAnalytical Line Shapes for Lorentzian Signals Broadened by Modulation,xe2x80x9d J. Appl. Phys. 36, 2522-2524 (1965)). The laser wavelength is modulated at a frequency xcexa9 with the modulation amplitude chosen such that the wavelength excursions are comparable to the width of the absorption line being investigated. The laser beam passes through the sample and impinges on a detector that provides a voltage or a current that is linearly proportional to the laser light power or intensity. The detector output is demodulated at the modulation frequency, or some integral multiple of the modulation frequency, to produce a signal that can be related to the sample absorbance. Demodulation methods are usually identified as 1f, 2f, 4f, etc., for demodulation at frequencies xcexa9, 2xcexa9, 4xcexa9, respectively. Demodulation using an odd harmonic, that is, 1f, 3f, etc. gives spectral waveforms that are typically zero when the laser wavelength is coincident with the gas absorption line center wavelength and that exhibit inversion symmetry about the line center wavelength. Detection using an even harmonic, that is 2f, 4f, etc., gives signals with extrema when the laser wavelength is at line center and these signal amplitudes are proportional to sample absorbance. FIG. 1 includes a representative absorption line spectrum, 10, as well as 1f, 2f, 3f and 4f spectral waveforms, 12, 14, 16, and 18, respectively.
To make practical the continuous, long term monitoring of the gas, the laser wavelength must be fixed at a wavelength within the absorption line of the gas. It is often preferred that the fixed wavelength coincide with the center of the absorption line. In the absence of active control of the laser wavelength, the laser wavelength will vary due to changes in the laser temperature, the laser gain profile, etc. Diode laser wavelengths can drift by an unacceptably large amount over time periods of less than 10 minutes. A number of schemes exists that use a selected absorption line of the target gas as a wavelength standard for controlling the laser wavelength. These techniques are known as line-locking methods and are well described by White (A. D. White, xe2x80x9cFrequency Stabilization of Gas Lasers,xe2x80x9d IEEE Journal of Quantum Electronics QE-1, 349-357 (1965)), with improvements to the art presented by Brun (Henri Brun, xe2x80x9cArrangement for Controlling the Frequency of a Light Source Using an Absorption Cell,xe2x80x9d U.S. Pat. No. 3,609,583, issued Sep. 28, 1971), by Smith (Peter William Smith, xe2x80x9cApparatus for Stabilizing a Laser to a Gas Absorption Line,xe2x80x9d U.S. Pat. No. 3,742,382, issued Jun. 26, 1973), by Buhrer (Carl F. Buhrer, xe2x80x9cFrequency Stabilization System,xe2x80x9d U.S. Pat. No. 3,593,189, issued Jul. 13, 1971) and by Kavaya (Michael J. Kavaya and Robert T. Menzies, xe2x80x9cSpectrophone Stabilized Laser with Line Center Offset Frequency Control,xe2x80x9d U.S. Pat. No. 4,434,490, issued Feb. 28, 1984). In each invention, a portion of the laser beam is directed through a reference cell holding a known amount of the gas being studied and then onto a detector. The laser wavelength is modulated by a small amount about its nominal wavelength and this modulation causes synchronous changes in the detector output. The usefulness of the modulation scheme is evident from FIG. 1 which includes a representative absorption line 10, i.e., absorbance plotted against laser wavelength. If wavelength modulation amplitude is comparable to the wavelength width of the absorption line and the detector output is processed using a phase sensitive detector, then the resulting spectral waveform looks like a 1f spectral waveform 12. The signal is zero when the laser average wavelength matches the absorption line center and it varies linearly with small displacements in wavelength about the line center. The signal can be used as a discriminant to correct the laser average wavelength back to the center of the absorption line.
It is the intent of most laser stabilization schemes to obtain the smallest possible fluctuations in the laser wavelength and, in many cases, demonstrated root mean squared wavelength fluctuations are as small as 1 part in 1010 to 1012. For example, both Brun and Smith use the method of saturated absorbance to achieve reference line widths considerably smaller than the line widths exhibited by the same reference gas in a conventional absorption measurement. These narrow line widths provide more precise control of the laser wavelength. The magnitude of the wavelength excursions required to implement wavelength modulation spectroscopy are at least as large as the absorber gas Doppler linewidth, which is larger than 1 part in 107 for nearly all gaseous absorbers.
The wavelength stabilization method disclosed by Cook is only applicable to lasers in which the output power as a function of wavelength exhibits the phenomenon known as a xe2x80x9cLamb dip.xe2x80x9d Cook""s invention is applicable only to some gas lasers in which the extent of continuous wavelength tunability is defined by the Doppler profile of an optical transition of a known gaseous component of the laser gain medium. Similarly, Fork""s laser stabilization method (R. L. Fork, xe2x80x9cFrequency Stabilized Optical Maser,xe2x80x9d U.S. Pat. No. 3,395,365, issued Jul. 30, 1968) is also limited to lasers making use of an active medium characterized by a Doppler broadened optical emission line.
Kavaya and Mead each disclose methods for stabilizing a laser to an absorption line at a wavelength different from the laser center wavelength. These wavelength offset approaches also use only one modulation frequency and are not useful for absorption measurements of a sample containing an unknown amount of the referenced gas.
Additional art describes laser wavelength stabilization schemes in which the laser has a stabilized output spectrum that is free of modulation. For example, Forster (Donald C. Forster, xe2x80x9cLaser Having a Stabilized Output Spectrum,xe2x80x9d U.S. Pat. No. 3,471,803, issued Oct. 7, 1969) describes a method in which the output wavelength of the laser to be stabilized is compared to the time-varying wavelength of a second laser whose output is modulated. Time-gated measurement of the wavelength difference between the two lasers provides an error signal that is used to control the wavelength of the unmodulated laser. In contrast, the present invention makes it desirable that the laser wavelength be modulated because wavelength modulation is required to measure small absorbances of the sample.
Additional prior art describes wavelength stabilization to an arbitrary wavelength that need not coincide with the wavelength of a specific absorption line of a reference gas. Itzkan (Irving Itzkan and Charles T. Pike, xe2x80x9cLaser Wavelength Stabilization,xe2x80x9d U.S. Pat. No. 3,967,211, issued Jun. 29, 1976) stabilizes the output of a wavelength tunable laser using a Fabry Perot etalon filter. Hall (John L. Hall and Miao Zhu, xe2x80x9cMethod and Apparatus for Laser Control,xe2x80x9d U.S. Pat. No. 4,856,009, issued Aug. 8, 1989) modulates the wavelength of the laser light using acousto-optic modulators that are external to the laser. An interferometer, which is similar in design to a Michelson interferometer, provides a phase discriminant that is used to control the laser wavelength. Both Itzkan and Hall describe methods that are less useful than are line locking methods for stabilizing a laser wavelength when the laser will be used for absorbance measurements.
The combination of line locking schemes and wavelength modulation spectroscopy suggests a method for continuous monitoring of a selected species in which line locking maintains the laser average wavelength coincident with the center of an absorption line of the target gas while the absorbance of a sample can be determined using demodulation at an even harmonic of the modulation frequency. Unfortunately, measurement of absorbance as defined by the magnitude of the demodulated sample signal will include fluctuations in the baseline, where the true baseline level is the demodulated detector output measured in the absence of absorbance. The instantaneous absorbance signal deviates from the true value due to the superposition of the baseline fluctuations on the absorbance signal. Fluctuations can be caused by electronic noise, vibration, etc., and the temporal bandwidth of such baseline fluctuations is typically below 1 kHz with significant variations occurring on a timescale of several seconds to several minutes. Baseline fluctuations can exceed the magnitude of the absorbance signal. It is possible to switch the laser wavelength periodically between the signal peak and a baseline region far from the absorption line in order to measure the baseline, but this scheme is not practical when line-locking is also required. Most experimental protocols for using wavelength modulation spectroscopy, or similar techniques, to quantify weak absorbances include means to scan the nominal, i.e., unmodulated, laser wavelength over a wavelength range that is substantially larger than the absorbance linewidth. The full contour of the harmonic waveform is recorded, permitting determination of the baseline value as well as the amplitudes of the extrema. Instrumentation, such as transient waveform averagers, needed to acquire the full spectral waveform on a timescale that is unperturbed by baseline fluctuations is expensive. Also, the overall measurement response time is reduced because the laser wavelength is not coincident with the absorption line during a significant portion of each measurement period.
Frequency modulation (FM) spectroscopy, described by G. C. Bjorklund (U.S. Pat. No. 4,297,035) is similar to wavelength modulation spectroscopy except that the FM method stipulates modulating the laser at frequency that is comparable to, or larger than, the linewidth of the absorption feature. This modulation produces discrete sidebands symmetrically distributed about the nominal laser frequency and differs from the modulation conditions used for wavelength modulation spectroscopy which generate a continuum of sidebands. In FM spectroscopy, demodulation at the modulation frequency (or higher harmonics) is possible, resulting in spectral lineshapes similar to those shown in FIG. 1. In practice, though, detection at higher harmonics is rarely used with FM spectroscopy due to detector bandwidth limitations.
Frequency modulation spectroscopy is not practical when the optimum modulation frequency exceeds the bandwidth of available detectors. An improvement to FM spectroscopy, two-tone FM spectroscopy, provides some of the advantages of FM spectroscopy while allowing the use of commercially available photodetectors and pre-amplifiers. The two-tone method is described by U.S. Pat. Nos. 4,594,511 and 4,765,736 and by D. E. Cooper and T. F. Gallagher, Applied Optics 24, 1327-1334 (1985). Modulation at two frequencies generates groups of sidebands. Sample absorbance is measured by demodulating the detector output at a frequency corresponding to a difference frequency between pairs of sidebands occurring within one of said groups. The most commonly used embodiment of two-tone FM spectroscopy generates spectral waveforms that are similar in shape and symmetry to a 2f waveform, as in trace 14 of FIG. 1(c). Two-tone FM spectroscopy differs from the present invention in that the two-tone FM method requires laser modulation at two frequencies and uses one demodulation step in order to obtain a signal proportional to sample absorbance whereas in the present invention only one modulation frequency is required to make an absorbance measurement that is equivalent in information content to the measurement made using two-tone FM spectroscopy. None of the published descriptions of two-tone FM spectroscopy include provisions for laser wavelength stabilization. The advantages and benefits of the present invention can be applied to the two-tone FM technique through the addition of a third modulation frequency and a second demodulation step.
Wavelength modulation spectroscopy including the use of two modulation frequencies has been described by Cassidy and Reid (D. T. Cassidy and J. Reid, Applied Physics B 29, 279-285 (1982)). The second modulation frequency provides a method for reducing signals from optical interference fringes. The variations in laser wavelength caused by the second modulation xe2x80x9csmears outxe2x80x9d the fringes, so that the unwanted signals average to zero. The Cassidy and Reid work differs from the present invention in that only one demodulation is performed; the absorbance signals are susceptible to the sources of baseline noise described above. Cassidy and Reid obtain their optimum result, i.e., the largest ratio of absorbance signal to interference fringe signal, when the higher modulation frequency is an integral multiple of the lower frequency. The present invention would perform poorly given this relationship of modulation frequencies. Also, Cassidy and Reid make no reference to laser wavelength stabilization.
The present invention relates to a method and apparatus for dual modulation of an optical spectroscopy laser. The invention provides wavelength stabilization and improved precision and accuracy of optical absorbance measurements. The invention comprises producing a light beam with a light source; modulating a wavelength of said light source beam with a first and a second frequency (the first frequency being different or greater than the second frequency); and producing output signal(s) representative of a gas species quantity and useful for wavelength stabilization with a detector. The invention also provides for demodulating the detector output signal.
In the preferred embodiment, the light source is a laser, such as a diode laser. The light beam is split into a first portion and a second portion.
The detector comprises a reference detector which produces an output signal representative of a known quantity of the gas species and a sample detector which produces an output signal representative of an unknown quantity of the gas species. The detector may be a is single detector or several detectors which produce signals representative of known and unknown quantities of the gas species and for wavelength stabilization. The detector may also provide demodulation.
The preferred demodulator comprises a first demodulator for demodulating the reference detector output signal and a second demodulator for demodulating the sample detector output signal. Each demodulator may utilize a reference frequency and generate other frequencies, preferably harmonics of the reference frequency. The first demodulating frequency of the first and second demodulators may be greater than the second demodulating frequency of the first and second demodulators; the second demodulating frequency of the first demodulator may be equal to the second demodulating frequency of the second demodulator; or the first demodulating frequency of the second demodulator may be an integral multiple of the first demodulating frequency of the first demodulator. The first demodulator produces a discriminant signal to stabilize the wavelength of the light source. The second demodulator reduces baseline noise in a signal proportional to sample absorbance of the unknown quantity of the gas.
A primary object of the invention is to reduce baseline noise in the signal proportional to sample absorbance.
Still another object of the invention is to provide a discriminant for regulating laser wavelength.
One advantage of the present invention is continuous monitoring of the number density of a gaseous species.
Another advantage of the present invention is its provision of a relatively inexpensive method for quantifying weak optical absorbances with rapid time response.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.